SiS 315

1
SiS 315

• An introductory exploration of features of the
  SVGA graphics processor used in our classrooms
  workstations

2
Some SVGA examples

• Our GPU is manufactured by SiS
• Official programming info is unavailable
• Can find some unofficial info on internet
• We explore some useful SiS 315 features
• But remember other GPU chips are NOT
  hardware-compatible (so our software is unlikely
  to work on other SVGA systems)
• We sacrifice the advantage of portability

3
SiS Extended Registers

• The typical Super VGA chip implements a number of
  extra hardware registers
• Most often these appear as additions to the
  standard set of VGA registers established by
  IBMs engineers
• Graphics Controller (9 registers)
• Timer-Sequencer (5 registers)
• CRT Controller (25 registers)
• Attribute Controller (21 registers)
• Our SiS graphics processor implements extra
  Sequencer registers (ports 0x3C4/0x3C5) and also
  extra CRTC registers (0x3D4/0x3D5)

4
SiS Sequencer registers

• Five standard VGA Sequencer-registers
• 0 the Reset register
• 1 the Clocking Mode register
• 2 the Map Mask register
• 3 the Character Map Select register
• 4 the Memory Mode register
• SiS implements registers 5, 6, 7, , etc.

5
In-Class Exercise

• Investigate the functionality of register 5
• Two-step process to read this register outb(
  5, 0x3C4 ) int val inb( 0x3C5 )
• Two-step process to write this register outb(
  5, 0x3C4 ) outb( val, 0x3C5 )
• Alternatively outw( (valltlt8)5, 0x3C4 )

6
Write a demo program

• Remember to call function iopl( 3 )
• Then create a program-loop
• For every possible byte-value
• write that byte-value to Seq-register 5
• read the byte-value in Seq-register 5
• print out the value that was read back
• What do you learn from this exercise?

7
Some further experiments

• Repeat the previous experiment, but add a new
  step within your program-loop
• 4) read (and print) Seq-register 6
• What do you learn from THIS exercise?
• OK, try it again, but with Seq-register 7 in
  place of Seq-register 6 (Caution Dont try to
  write new values into registers 6 or 7)

8
Detecting screen-height

• The screen-height parameter is 11-bits
• Lowest 8-bits are in CRTC register 0x12
• 9th and 10th bits are in CRTC register 0x07
• This much is standard on VGA hardware
• 11th bit is in SiS Sequencer register 0x0A
• We need to read all three registers, then extract
  and assemble the various bits

9
The screen-height bits
crtc-0x12 (Vert Displ Enable End)
crtc-0x07 (Overflow Register)
screen_height - 1
?
seq-0x0A
10
Detecting screen-width

• The screen-width parameter is 10 bits
• It measures the width in 8-pixel units
• Lowest 8-bits are in CRTC register 0x01
• 9th and 10th bits in Extd Seq register 0x0B
• The parameter-value is actually one less
• It gives the id-number of the final unit
• So we increment it, then multiply by 8

11
The screen-width bits
crtc-0x01 (Horizontal Display End)
screen_width 1 (in 8-pixel units)
seq-0x0B
12
Detecting CRT start-address

• Very important parameter for animation!
• This parameter is stored as 26 bits
• It measures start-address in 4-byte units
• Lowest byte stored in crtc register 0x0D
• Next lowest byte is in crtc register 0x0C
• Sequencer register 0x0D holds next byte
• Sequencer register 0x37 has 2 more bits

13
The crt start-address bits
crtc-0x0D (Start Addr Lo)
crtc-0x0C (Start Addr Hi)
crt start-address (in 4-byte units)
seq-0x0D
seq-0x37
14
Detecting CRT Line-Compare

• CRTC-address resets when counter LC
• This is useful for certain special effects
• Screen is split into two separate windows
• Lower window shows VRAM address 0
• Upper window is from crt start-address
• Lowest 8-bits from CRT register 0x18
• 9th bit from CRT register 0x07
• 10th bit from CRT register 0x09
• More bits from Sequencer register 0x0F

15
The Line-Compare bits
crtc-0x18 (Line Compare register)
crtc-0x07 (Overflow register)
Line-Compare
crtc-0x09 (Max Scan Line)
seq-0x0F
16
In-Class Experiment

• Reprogram the Line Compare parameter with a
  value that is equal to one-half of the current
  screen-height
• What visual effect do you observe?
• Do you see a similar effect under Linux in both
  text mode and in graphics modes?